1. Field of the Invention
The invention relates to interference lithography (IL), also known as interferometric lithography or holographic lithography, a method wherein periodic or quasi-periodic patterns are exposed into a photosensitive material (in thin-film form, usually called a resist), by overlapping pairs of phase-locked beams from a laser or other intense source of radiation. In general, a beam, such as emitted from a laser, is split into pairs of beams, which are then directed to recombine at a resist-coated substrate. At the intersection of the beams a periodic interference pattern is created with period p=λ/(2 sin θ), where λ is the beam wavelength and θ is intersection half-angle of a particular beam pair.
2. Description of Prior Art
A controlled phase relationship must hold between the two halves of a beam pair, and between all sets of beam pairs (i.e., they must be “phase-locked”) in order to form stable, high-contrast fringes. This can be achieved passively, by the use of a rigid, compact optical system designed for thermal and mechanical stability, or alternatively by the use of active optical components and phase-error measuring sensors in conjunction with phase-locking feedback electronics (see E. H. Anderson, H. I. Smith, and M. L. Schattenburg, “Holographic Lithography,” U.S. Pat. No. 5,142,385).
When a coherent source of light is used, such as a laser, the beam is typically split with a dielectric beamsplitter and recombined with mirrors (see H. I. Smith, “Fabrication techniques for surface acoustic wave and thin-film optical devices,” Proc. IEEE 62, 1361-1387 [1974]). In an alternative configuration, known as achromatic interference lithography (AIL), the beam is split and recombined using diffraction gratings (see T. A. Savas, M. L. Schattenburg, J. M. Carter, and H. I. Smith, “Large-area achromatic interferometric lithography for 100 nm period gratings and grids,” J. Vac. Sci. Technol. B 14, 4167-4170 [1996]). In this second case, beams lacking high spatial and temporal coherence can still be used to make useful gratings. The AIL method utilizes much more compact optics that the IL method, resulting in a very stable interferometer. However, a disadvantage is that each AIL system can pattern only a single period.
IL has been used commercially for many years to produce large-area diffraction gratings for spectroscopy. Other industrial and research applications for IL-patterned gratings and grids include: optical components for filtering, polarizing, diffracting and other manipulations of light, x-rays, and particle beams; length-scale standards for metrology; positional encoders in motion control equipment; fiducial grids used during spatial-phase locked electron-beam lithography (see H. I. Smith, E. H. Anderson, and M. L. Schattenburg “Energy beam locating,” U.S. Pat. No. 5,136,169); arrays of field emitter tips for flat panel display manufacturing (see C. O. Bozler et al., “Arrays of gated field-emitter cones having a 0.32 μm tip-to-tip spacing,” J. Vac. Sci. Technol. B 12, 629 (1994)); and high density magnetic storage (see M. Farhoud et al., “Fabrication of large area nanostructured magnets by interferometric lithography,” IEEE Trans. Mag. 34, 1087-1089 (1998)).
Competitive non-IL means of producing precision periodic patterns'suffer from a number of well-known deficiencies. For example, the technique of mechanical ruling suffers from extremely slow speed, poorly-controlled grating groove profile, inability to pattern very fine periods, distortions due to limited servo-loop gain, and incompatibility with semiconductor lithographic processing. The method of energy beam writing (e.g., electron, beam lithography or ion beam lithography) suffers from slow speed and large grating distortions due to poor beam positioning control (e.g., stitching errors).
In general, IL methods are presently superior to competing methods for rapidly producing precision periodic patterns. In current IL practice, large-area patterns are generally achieved by expanding the beams with lenses, after which they are caused to overlap and interfere. Beams with spherical wavefronts can be achieved by using a short-focal-length lens followed by an (optional) spatial-filter pinhole at the lens focus. However, the interference of spherical beams produces gratings with large hyperbolic distortions. Gratings substantially free of hyperbolic distortions can be achieved by following the spatial filter with a second, collimating lens. However, in this case, distortions in the collimating lens due to inevitable manufacturing errors are directly translated into distortions in the grating. In addition, the collimating lens must be at least as large as the substrate being patterned. Thus, for good results, an IL system with large, precisely figured optics and/or very long optical paths is required. Such a system is bulky, expensive, and vulnerable to the distorting effects of vibration, air turbulence, and thermal fluctuations. Uniform exposure dose is also difficult to achieve. It is also difficult, expensive, and time consuming to reconfigure such a system in order to fabricate other types of general periodic patterns such as gratings with other periods, grids (crossed gratings), and “chirped” gratings with variable periods.
The AIL method, on the other hand, avoids the need for a highly coherent source and is also more stable than the traditional IL method due to its compact design, but does require splitter and recombiner grating optics of superb quality which are at least as large as the desired substrate size. In addition, the AIL method is even less flexible that the IL method for patterning general periodic patterns, since each AIL interferometer is designed to pattern only one period.
Thus, current practice does not allow the rapid and low-cost patterning of large, low distortion, general periodic and quasi-periodic patterns with highly uniform and controlled properties. The object of this invention is to provide these benefits by utilizing novel means of conducting IL with phase-locked scanning beams.